Floating aerator

ABSTRACT

A floating liquid aerator includes an axial flow propeller rotated by an electric motor and disposed within a tubular throat to pump water from a pond upwardly through the throat and against a rotatable diffuser which slings the water outwardly for interface contact with the surrounding atmosphere. Telescoped into the throat and over the propeller is a tubular ring duct which divides the upwardly flowing column of water into a central core and a surrounding outer cylinder to prevent the formation of a vortex within the throat and thereby increase the pumping efficiency of the aerator. More nearly laminar flow across the propeller is produced by a series of stationary infuser vanes positioned beneath the ring duct to feed the inflowing water inwardly to the propeller at a controlled angle. Increased transfer of oxygen to the water is achieved by introducing air into the upwardly flowing water, the air being admitted into the outer cylinder of water in the area of the ring duct to avoid cavitation at the propeller.

FOREIGN PATENTS 1,100,569 3/1961 Germany......................

Primary Examiner-Ronald R. Weaver AttorneyWo1fe, Hubbard, Leydig, Voit and Osann ABSTRACT: A floating liquid aerator includes an axial flow propeller rotated by an electric motor and di tubular throat to pump water from a the throat and against a rotatable 2 259/95 water outwardly for interface contact with the surroun Int. mosphere Telescoped into the throat and over the a tubular ring duct which divides the u of water into a central core and a sur to prevent the formation of a vortex within the throat and thereby increase the pumping efficien nearly laminar flow across the prope ries of stationary infuser vanes duct to feed the inflowing water Richard B. Ravitts Rockford, 111.

Jan. 15, 1968 Mar. 30, 1971 Richards of Rockford, Inc. Rockford, Ill.

10 Claims, 5 Drawing Figs.

[ B0lf5/l2, B01f3/04, C02c 5/00 261/36, 91, 93,120;259/97,95;209/169;210/15,194,197, 220, 221,219; 239/16 References Cited UNITED STATES PATENTS 2,239,194 4/1941 Fitzgerald et a1. 2,120,786 6/1938 Jo Inventor United States Patent [21] App1.No. 697,681

[22] Filed [45] Patented [73] Assignee [54] FLOATING AERATOR controlled angle. Increased transfer of o achieved by introducing air into the u the air being admitted into the outer area of the ring duct to avoid cavitation at the propeller.

w r m K 6 6 9 1 8 9 0 9 6 3 3,416,729 12/1968 Ravittsetal..............::::

Paten ed March 30, 1971 3,572,658

2 Sheets-Sheet 1 Patented March 30, 1971 3,572,658

2 Sheets-Sheet 2 v I 4 WM OQTTQIQMEYJ FLOATING AERATOR BACKGROUND OF THE INVENTION This invention relates to a liquid aerator of the type typically used in ponds or tanks to enrich the oxygen content of water or other liquid, to scrub undesirable gases from the water, or to cool large quantities of water for recirculation. More particularly, the invention constitutes an improvement over an aerator such as disclosed in Ravitts U.S. Pat. No. 3,416,729 in which the water is pumped upwardly from a pond through an upright tubular throat by a rotatable propeller and then is slung outwardly for interface contact with the surrounding air prior to falling back into the pond. The propeller is power rotated by an electric motor and comprises a series of angularly spaced and radially extending blades which are twisted to attack the water at an angle which progressively decreases from the roots to the tips of the blades so that a substantially constant volume of flow is developed at all points along the length of each blade.

SUMMARY OF THE INVENTION The present invention aims primarily to provide a new and improved liquid aerator of the above character which not only reduces the danger of overloading and burning out the drive motor when the unit is pumping at full capacity but which also is capable of circulating a greater volume of water for each unit of power developed by the motor than prior aerators of the same general type. In particular, overloading of the motor is reduced and the pumping efficiency is increased by the provision of a tubular ring duct coacting with the throat to establish a flow pattern to and from the propeller in opposition to the swirling vortical pattern normally created at the intake of an axial flow pump. The ring duct destroys and prevents the formation of a vortex at the intake thereby decreasing turbulence and drag resisting upward flow of the water. Moreover, the ring duct causes the inducement of a relatively low velocity flow of water through a bypass area surrounding the main column of water being drawn upwardly by propeller and results in a significant increase in the total volume of water pumped in a given time by an aerator with a motor of given capacity.

A further aim of the invention is to provide a series of novel infuser vanes beneath the propeller to feed the water inwardly to the propeller blades at a controlled angle in order to produce more nearly laminar flow across the propeller and thereby increase still further the efficiency of the aerator. Advantageously, the infuser vanes are twisted in accordance with the twist of the propeller blades so that the angle at which the water is fed to the blades is correlated with the attack angle of the blades along the entire length of the latter to reduce drag and turbulence.

The invention also resides in the introduction of air bubbles into the bypass flow area to increase the oxygenation of the water while avoiding cavitation at the intake of the aerator.

These and other objects and advantages of the invention will become apparent as the following description proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a side elevation of an aerator embodying the novel features of the present invention, parts being broken away and shown in section;

FIG. 2 is an enlarged view of parts shown in FIG. 1;

lFlG. 3 is an enlarged fragmentary cross section taken substantially along the line 3-3 of FIG. 2;

FIG. 4 is an enlarged fragmentary cross section taken substantially along the line 4-4 of FIG. 2; and

FIG. 5 is an enlarged fragmentary cross section taken substantiallyalong the line 5-4 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawings for purposes of illustration, the invention is embodied in a liquid aerator 10 adapted to float in a body of liquid such as water in a tank, a pool or a waste treatment pond and serving to enrich the oxygen content of the water while scrubbing the latter of undesirable gases. Alternatively, the aerator may be used to cool heated water before such water is recirculated through various types of systems.

Herein, the aerator I0 is specifically shown as being used in a waste treatment pond and acts to scrub and oxidize the water therein by continuously drawing water upwardly out of the pond and then slinging the water into the surrounding atmosphere for interface contact with the air before falling back to the surface of the pond. The water is slung outwardly from the aerator with considerable turbulence so that noxious gases in the water are released and are replaced by oxygen from the atmosphere.

More specifically, the floating aerator I0 is carried by a support comprising a buoyant float 11 formed by a stainless steel outer shell 13 which is filled with a low density material 14 such as polyurethane foam. The float may be moored in the pond by attaching cables (not shown) to eyes 15 angularly spaced around the periphery of the shell.

Extending vertically through the center of the float III is a cylindrical opening into which is telescoped an upright tubular throat l6 defining a passage accommodating the flow of water upwardly out of the pond and into the atmosphere. At its lower end, the throat is formed with an outwardly flaring intake shroud l7 immersed in the water and establishing an entryway into the throat. The upper end of the throat is open and is located above the float to discharge the water into the atmosphere.

Water is pumped upwardly from the pond and through the throat 16 for interaction with the air by an axial flow propeller 19 telescoped into the throat and fastened to the lower end of a rotatable propeller shaft 20 extending axially into the throat and coupled at its upper end to the drive shaft of an electric motor 21. The latter is mounted on a platform 23 which is supported above the upper surface of the float 11 by a plurality of upstanding legs 24 angularly spaced from each other and establishing wide flow passages between the float and the platform for the outward discharge of the water.

As the water emerges from the upper end of the throat 16, it impinges against a rotatable diffuser 25 (FIG. 1) and is slung upwardly and outwardly above the upper side of the float I1 and is dispelled over a wide area. The diffuser 25 is fastened to and rotatable with the propeller shaft 20 just below the platform 23 and preferably is formed on its lower side with an up wardly and outwardly tapering truncated conical surface 26 against which the upwardly rising water impinges. The water slung outwardly by the diffuser is churned and mixed with the air with a vigorous scrubbing action thereby producing a high rate of oxygen transfer from the air to the water.

To stabilize the float 11 against rocking in the water and to reduce vibration transmitted to the motor 21, the propeller 19 is statically, dynamically and hydrodynamically balanced. In this instance, the propeller comprises four identical blades 27 angularly spaced around and radiating outwardly from a central hub 29 fastened to the lower end of the propeller shaft 20. Balancing is achieved by spacing the blades equally around the hub and by twisting the blades along their long'tudinal centerlines so that the same volume of flow is produced at all points along the length of the blades. As shown in FIG. 4, each blade is fastened to the hub with its inner end or root Sltl inclined from a horizontal plane at a predetermined attack angle ra which herein is approximately 4. Because the outer end or tip 31 of the blade rotates at a higher speed than the root 30, the blade is twisted such that the angle with which the blade attacks the water decreases progressively from the root to the tip. Such twisting of the blades results in each blade moving substantially the same volume of water across its entire length even though the velocity of the blade varies from its root to its tip. In one particular instance, the blade is twisted to establish an attack angle ta (FIG. 3) of about 25 at the tip 31 when the attack angle ra of the root is 40. The attack angle of the intennediate blade portions between the root and the tip decreases progressively and preferably linearly between the two extreme values as the blade progresses outwardly from the root.

In accordance with one aspect of the present invention, the pumping efliciency of the aerator is increased and the tendency of the drive motor 21 to overload is reduced by establishing within the throat 16 a flow pattern which opposes the swirling vortical pattern normally created within an axially pumped column of water. For this purpose, a tubular ring duct 35 is telescoped into the throat and over the propeller 19 to set up a pressure and velocity pattern within the throat effective to prevent the formation of a vortex either above or below the propeller. This reduces turbulence which is normally organized within and created by a vortex and minimizes drag losses through the throat and across the propeller thereby resulting in a greater volume of flow at a higher velocity.

Herein, the ring duct 35 is simply a cylindrical sleeve coaxial with the propeller 19 and fastened to the throat 16 by six angularly spaced gussets 36 (FIG. 5) welded to the outer wall 37 of the ring duct and the inner wall of the throat. The ring duct is centered axially of the propeller 19 and is of sufficient axial length to extend both above and below the blades 27 so that the latter are substantially surrounded by the duct. As shown most clearly in FIGS. 2 and 5, the interior wall 39 of the ring duct 35 is spaced outwardly from the tips 31 of the blades by a very short distance (herein, about one thirty-second of an inch) while the outer wall 37 of the duct is spaced a considerably greater distance from the inner wall of the throat 16, the latter spacing in this instance being approximately threefourths of an inch.

With this arrangement, the interior wall 39 of the ring duct defines a primary flow passage through which a main column or central core of water is drawn upwardly at high velocity by the propeller 19 as the latter rotates. At the same time, the upwardly flowing central core of water passing directly across the propeller induces an indirect upward flow of a secondary column or cylinder of water through an annular bypass area 40 (FIGS. 2 and 5) surrounding the central core and established by the radial spacing between the outer wall 37 of the ring duct and the inner wall of the throat 16. Thus, the ring duct divides the upwardly flowing water into two streams, that is, a primary stream or central core passing through the duct and across the propeller and a secondary stream or outer cylinder surrounding the central core and induced to flow along the outer wall of the duct.

As a result, the central core of water is sheared from the outer cylinder by the duct 35 and passes upwardly within the outer cylinder at a high velocity rather than flowing alongside the inner wall of the throat 16 and being subjected to losses inherently occurring from frictional drag of the water along the inner wall. Thus, instead of pumping the central core along the wall against a high Reynolds number, the propeller 19 works only against the shear coefficient of the water at the interface of the central core to the outer cylinder of water. Accordingly, the inner and outer portions of the core move at approximately the same speed and a flow with a substantially uniform cross-sectional pressure and velocity pattern is created through the duct 35 and across the propeller to minimize energy losses caused by drag and turbulence.

By virtue of the drag created by the shear coefficient of the water at the interface of the pumped central core to the surrounding outer cylinder, the latter is induced to flow upwardly with the main core and passes through the bypass area 40 and along the inner wall of the throat 16. Because the bypass flow is induced, the outer cylinder move along the inner wall of the throat at a velocity which is slower than that of the core and, since the Reynolds number decreases approximately by the cube in relation to decreases in velocity, the outer cylinder is pumped upwardly against a Reynolds number which is smaller than outerwise would be the case if the high velocity central core were pumped directly along the inner wall of the throat. The high velocity central core and the slower moving outer cylinder create a pressure and velocity pattern across the entire throat effective to counteract the pattern of a vortex and, as a result, power losses caused by drag are reduced and a greater volume of flow. with less turbulence is developed. ln addition, the ring duct forms a hydrodynamic fence" around the tips of the blades to prevent secondary vortical flow around the tips thereby contributing to laminar flow.

In another aspect, the present invention contemplates feeding the inflowing water to the propeller 19 at a controlled angle to produce a more nearly laminar flow across the propeller and thereby further increase the efficiency of the aerator 10. To these ends, a series of stationary infuser vanes 43 (FIG. 2) are positioned in the throat 16 directly beneath the propeller to deflect the inflowing water to the propeller blades 27 at an angle which changes along the length of each blade in accordance with changes in the attack angle of the blade. Thus, the water is fed to the blade at an angle changing uniformly along the length of the blade to reduce losses caused by turbulent flow.

As shown most clearly in FIGS. 2 and 5, the aerator 10 herein includes three such infuser vanes 43 spaced angularly from each other by equal distances and extending radially of the throat 16 just below the ring duct 35. At their inner ends or roots 44, the three vanes are welded together into a rigid unit. The outer ends or tips 45 of the vanes project radially beyond the ring duct and are welded to the inner wall of the throat.

Like the propeller blades 27, the infuser vanes 43 are twisted along their longitudinal centerlines with the attack angle of each vane relative to the horizontal decreasing from its root 44 to its tip 45. As shown in H0. 4. the attack angle vr of the root 44 of each vane is set in one particular instance at when the vanes are used in conjunction with propeller blades each having an attack angle ra of 40 at its root 30. Each vane is twisted by an amount equal to the twist placed in the propeller blades 27 so that the difference in the attack angle between the two corresponding and overlying points along the length of each propeller blade. Accordingly, the attack angle vl (FIG. 3) near the tip 45 of each infuser vane shown herein is equal to approximately 65.

With the foregoing arrangement, the infuser vanes 43 deflect the inflowing water inwardly into the propeller blades 27 at an angle which changes along the length of the propeller blades in accordance with their attack angle. Thus, the blades cut through the water with the latter meeting each lengthwise portion of the blades at a controlled angle to reduce turbulence and losses.

The addition of both the ring duct 35 and the infuser vanes 43 to the aerator 10 increases-its pumping efficiency from 33,000 gal/hp/hr to above 40,000 gal/hp/hr or a gain of over 7,000 gal/hp/hr. While the ring duct can be used independently of the infuser vanes, and vice versa, it has been found that the overall increase in efficiency attained when the duct and the vanes are used together is greater than the combined increases attained when each is used separately of the other.

Advantage is taken of the ring duct 35 to inject air bubbles into the upwardly flowing water without causing cavitation at the propeller 19. Herein, this is achieved by means of several conduits or tubes 50 F168. 1, 2 and 5) which extend upwardly along the outer side of the throat 16 with their upper ends 51 projecting upwardly through the platform 23 and being vented to the atmosphere. At their lower ends, the tubes project radially through the walls of the throat and open into the bypass area 40 as indicated at 53. As the water flows upwardly through the bypass area, air is sucked from the tubes with a venturi action and is introduced into the water to increase the oxygen transfer rate. Since the ring duct 35 isolates the propeller 19 from the bypass area 40, such advantageous introduction of air into the water at the bypass area can be accomplished without causing cavitation and without creating power and efficiency losses resulting from cavitation.

lclaim:

1. In a liquid aerator of the type comprising a float adapted to be supported buoyantly in a body of liquid and having a vertically extending opening formed therethrough, an upright tubular throat telescoped into said opening with its lower end projecting below the float for immersion in the liquid and with its upper end opening above'the float for the discharge of liquid pumped upwardly through the throat, a power rotated shaft carried by said float and extending axially into said throat, a propeller fixed to and rotatable with said shaft to pump liquid upwardly through said throat in response to rotation of said shaft, said propeller comprising a series of angularly spaced and twisted blades radiating outwardly from said shaft and having tips spaced radially inwardly from said throat, and a rotatable diffuser fixed to said shaft above said propeller and said float for impinging against liquid moving upwardly through said throat and for slinging the liquid outwardly across the top of the float, the improvement in said aerator comprising, a tubular ring duct telescoped into said throat and over said propeller with its interior wall surrounding the tips of said blades, said ring duct being spaced inwardly from the walls of said throat to divide the liquid flowing into the throat into a central core passing through the ring duct and an outer cylinder surrounding the core and passing through the space between the ring duct and the throat, and a series of angularly spaced infuser vanes extending radially beneath said ring duct and anchored in stationary positions,said infuser vanes being twisted by an amount correlated with the twist of said propeller blades to deflect the liquid into the latter at a controlled angle.

2. An aerator as defined in claim 1 further including at least one air conduit having an upper end vented to the atmosphere above the liquid, said conduit extending downwardly along said throat and having a lower end opening into the space between said ring duct and said throat to introduce air into the outer cylinder of liquid.

3. In a liquid aerator, the combination of, a support having an upright tubular throat adapted for immersion in a body of liquid at its lower end and defining an opening at its upper end for the discharge of liquid pumped upwardly through the throat, a power rotated shaft carried by said support and extending axially into said throat, a propeller fixed to and rotatable with said shaft within said throat to pump liquid upwardly through the throat in response to rotation of the shaft, said propeller comprising a series of angularly spaced blades radiating outwardly from said shaft and having tips spaced radially inwardly from said throat, tubular ring duct telescoped into said throat and over said propeller with the ring duct being disposed entirely within said throat and with the blades being disposed entirely within the ring duct, the interior wall of said ring duct surrounding the tips of said blades from the top to the bottom thereof and defining a primary flow passage through which a column of liquid is drawn upwardly by the blades as the latter rotate, and said ring duct being spaced radially inwardly from said throat and defining with the latter a secondary flow passage through which a second column of liquid is induced to flow as a result of the liquid being drawn upwardly through said primary passage.

4. An aerator as defined in claim 3 in which said throat and said ring duct are of cylindrical cross section and are coaxial with said shaft, the radial spacing between said ring duct and said throat being greater than the radial spacing between the ring duct and the tips of said blades.

5. An aerator as defined in claim 3 further including a series of angularly spaced infuser vanes extending radially across said primary flow passage beneath said propeller and stationarily anchored within said throat to direct upwardly flowing liquid to said propeller blades at a controlled angle.

6. An aerator as defined in claim 5 in which each propeller blade is twisted about its longitudinal centerline with the attack angle of the blade decreasing progressively from its root to its tip, and each of said infuser vanes being twisted about its longitudinal centerline in accordance with the twist of each propeller blade to direct the liquid to the latter at a controlled angle along the lenglth of the propeller blade.

. An aerator as efined in claim 3 further including at least one air conduit having an upper end vented to the atmosphere above the liquid, said conduit extending downwardly along the outer side of said throat and communicating at its lower end with said secondary flow passage to introduce air into said second column of water.

8. In a liquid aerator, the combination of, a support having an upright tubular throat adapted for immersion in a body of liquid at its lower end and defining an upright passage through which liquid is pumped upwardly for discharge at the upper end of the throat, a power rotated shaft carried by said support and extending axially into said throat, a propeller fixed to and rotatable with said shaft within said throat to pump a column of liquid upwardly through said passage in response to rotation of said shaft, said propeller having a hub fixed to said shaft and having a series of angularly spaced blades radiating outwardly from the hub with their roots fixed to the hub and with their tips spaced radially inwardly from said throat, each blade being twisted with a progressively decreasing attack angle from its root to its tip and a series of angularly spaced infuser vanes joined together at their inner ends and radiating outwardly toward said throat beneath said propeller blades with their outer ends fixed to said throat, each of said infuser vanes being twisted with a progressively decreasing attack angle from its inner end to its outer end to direct the flow of liquid to each propeller blade at an angle which constantly changes along the length of the blade in accordance with changes in the attack angle of the blade.

9. An aerator as defined in claim 8 in which the difference in the attack angle between any two points along the length of each propeller blade is equal to the difference in the attack angle between two corresponding and underlying points along the length of each infuser vane.

10. An aerator as defined in claim Sfurther including a tubular ring duct telescoped over said propeller and into said throat and fixed to the latter above said infuser vanes, said ring duct surrounding the tips of said propeller blades and being spaced radially inwardly from said throat to establish a selected pattern of liquid flow to the blades in opposition to the vortical flow pattern normally created in the liquid. 

1. In a liquid aerator of the type comprising a float adapted to be supported buoyantly in a body of liquid and having a vertically extending opening formed therethrough, an upright tubular throat telescoped into said opening with its lower end projecting below the float for immersion in the liquid and with its upper end opening above the float for the discharge of liquid pumped upwardly through the throat, a power rotated shaft carried by said float and extending axially into said throat, a propeller fixed to and rotatable with said shaft to pump liquid upwardly through said throat in response to rotation of said shaft, said propeller comprising a series of angularly spaced and twisted blades radiating outwardly from said shaft and having tips spaced radially inwardly from said throat, and a rotatable diffuser fixed to said shaft above said propeller and said float for impinging against liquid moving upwardly through said throat and for slinging the liquid outwardly across the top of the float, the improvement in said aerator comprising, a tubular ring duct telescoped into said throat and over said propeller with its interior wall surrounding the tips of said blades, said ring duct being spaced inwardly from the walls of said throat to divide the liquid flowing into the throat into a central core passing through the ring duct and an outer cylinder surrounding the core and passing through the space between the ring duct and the throat, and a series of angularly spaced infuser vanes extending radially beneath said ring Duct and anchored in stationary positions, said infuser vanes being twisted by an amount correlated with the twist of said propeller blades to deflect the liquid into the latter at a controlled angle.
 2. An aerator as defined in claim 1 further including at least one air conduit having an upper end vented to the atmosphere above the liquid, said conduit extending downwardly along said throat and having a lower end opening into the space between said ring duct and said throat to introduce air into the outer cylinder of liquid.
 3. In a liquid aerator, the combination of, a support having an upright tubular throat adapted for immersion in a body of liquid at its lower end and defining an opening at its upper end for the discharge of liquid pumped upwardly through the throat, a power rotated shaft carried by said support and extending axially into said throat, a propeller fixed to and rotatable with said shaft within said throat to pump liquid upwardly through the throat in response to rotation of the shaft, said propeller comprising a series of angularly spaced blades radiating outwardly from said shaft and having tips spaced radially inwardly from said throat, tubular ring duct telescoped into said throat and over said propeller with the ring duct being disposed entirely within said throat and with the blades being disposed entirely within the ring duct, the interior wall of said ring duct surrounding the tips of said blades from the top to the bottom thereof and defining a primary flow passage through which a column of liquid is drawn upwardly by the blades as the latter rotate, and said ring duct being spaced radially inwardly from said throat and defining with the latter a secondary flow passage through which a second column of liquid is induced to flow as a result of the liquid being drawn upwardly through said primary passage.
 4. An aerator as defined in claim 3 in which said throat and said ring duct are of cylindrical cross section and are coaxial with said shaft, the radial spacing between said ring duct and said throat being greater than the radial spacing between the ring duct and the tips of said blades.
 5. An aerator as defined in claim 3 further including a series of angularly spaced infuser vanes extending radially across said primary flow passage beneath said propeller and stationarily anchored within said throat to direct upwardly flowing liquid to said propeller blades at a controlled angle.
 6. An aerator as defined in claim 5 in which each propeller blade is twisted about its longitudinal centerline with the attack angle of the blade decreasing progressively from its root to its tip, and each of said infuser vanes being twisted about its longitudinal centerline in accordance with the twist of each propeller blade to direct the liquid to the latter at a controlled angle along the length of the propeller blade.
 7. An aerator as defined in claim 3 further including at least one air conduit having an upper end vented to the atmosphere above the liquid, said conduit extending downwardly along the outer side of said throat and communicating at its lower end with said secondary flow passage to introduce air into said second column of water.
 8. In a liquid aerator, the combination of, a support having an upright tubular throat adapted for immersion in a body of liquid at its lower end and defining an upright passage through which liquid is pumped upwardly for discharge at the upper end of the throat, a power rotated shaft carried by said support and extending axially into said throat, a propeller fixed to and rotatable with said shaft within said throat to pump a column of liquid upwardly through said passage in response to rotation of said shaft, said propeller having a hub fixed to said shaft and having a series of angularly spaced blades radiating outwardly from the hub with their roots fixed to the hub and with their tips spaced radially inwardly from said throat, each blade being twisted with a progressively decreasing attack aNgle from its root to its tip and a series of angularly spaced infuser vanes joined together at their inner ends and radiating outwardly toward said throat beneath said propeller blades with their outer ends fixed to said throat, each of said infuser vanes being twisted with a progressively decreasing attack angle from its inner end to its outer end to direct the flow of liquid to each propeller blade at an angle which constantly changes along the length of the blade in accordance with changes in the attack angle of the blade.
 9. An aerator as defined in claim 8 in which the difference in the attack angle between any two points along the length of each propeller blade is equal to the difference in the attack angle between two corresponding and underlying points along the length of each infuser vane.
 10. An aerator as defined in claim 8 further including a tubular ring duct telescoped over said propeller and into said throat and fixed to the latter above said infuser vanes, said ring duct surrounding the tips of said propeller blades and being spaced radially inwardly from said throat to establish a selected pattern of liquid flow to the blades in opposition to the vortical flow pattern normally created in the liquid. 